xTalk has had a complete overhaul and now uses the CKEditor for entries. This means that you can now upload images in topics and replies.
xTalk also makes use of the XPages Extension Library for DialogBoxes and the new Name Picker. The new name picker now means that the 64k limit on name lookups in the old version of xTalk are now a thing of the past.
Other notable features for developers to look at are the registration and 'forgot password' scripts. These use a combination of ExtLib dialog boxes and the new 8.5.2 feature sessionAsSigner so that you don't have to call out to an external agent to do the work.
The 'applicationInit' script also uses the sessionAsSigner to build a default configuration document in the database if none exists, this means that the database will run and open correctly on screen even if you access it with anonymous access rights for the very first time.